1. Field of the Invention
The present invention relates to an acoustic apparatus which is an open type but does not provide a sound to people other than a listener himself or herself in a mobile body such as an aircraft or in space in which a plurality of people are present.
2. Description of the Background Art
In general, when music or the like is enjoyed in an environment in which many people are present, like in a mobile body such as an aircraft, headphones are used to avoid a sound leaking to the nearby area and to allow only the headphone-wearing person to hear.
On the other hand, there is a system which provides music or the like by means of an open-type loudspeaker to alleviate a blocked-up feeling and an oppressive feeling which are caused by the headphones. However, it is impossible to differentiate, among the people being present in the range of hearing the sound from the loudspeaker, between those who want to hear the sound and those who do not want to hear the sound. Therefore, there is a directional acoustic apparatus which is capable of providing a sound to those who want to hear the sound. As a sound source of this directional acoustic apparatus, a loudspeaker using ultrasonic waves is used (for example, refer to U.S. Pat. No. 4,823,908 by the same applicant, which discloses a parametric loudspeaker).
For example, realized in patent document 1 is a system which includes a plurality of directional loudspeakers and a plurality of human body sensors. The system causes no sound to be provided to other people by emitting no sound to a place where nobody is present. In addition, proposed therein is the system which also includes distance sensors and causes information to be changed as needed in accordance with a distance.
In addition, proposed in patent document 2 is a system which allows a listener himself or herself to adjust an angle of a loudspeaker installed in a seat to make a sound inaudible to other people.
Furthermore, proposed in patent document 3 are human detection methods in which a human is thermally detected and in which positional information of a face is detected.    [Patent document 1] Japanese Patent Application Laid-Open Publication No. 2005-159446    [Patent document 2] Japanese Patent Application Laid-Open Publication No. 5-345549    [Patent document 3] Japanese Patent Application Laid-Open Publication No. 7-192589
However, the directional loudspeaker used in each of the above-mentioned patent documents and the like has characteristics in which a sound emitted therefrom travels greatly straight forward and the sound is not attenuated until the sound reaches a remote place. This causes the sound to reach anywhere if there is no obstacle and to be heard by people being present in such a direction.
In addition, because of the characteristics that the sound emitted therefrom travels straight forward, if there is an object having reflecting properties, the sound is reflected by the object and an audible area different from the area to which the sound is originally directed is made.
For example, a directional loudspeaker installed in a seat has a problem in that when an obstacle or a person is not present, the sound is heard by other people (somebody passing nearby and people sitting behind) present on a straight line extending from the directional loudspeaker. In addition, when an obstacle is present, ultrasonic signals may be repeatedly reflected by the obstacle and the sound may be heard by other people.
In addition, a position or an outputting direction of the loudspeaker is likely to be changed by an operation of reclining a front seat irrespective of an intention of a listener himself or herself of the loudspeaker. Therefore, unless outputting of a sound is automatically stopped or reduced before the listener of the loudspeaker adjusts outputting of the sound, a nuisance to other people may be caused.
Moreover, when an obstacle suddenly appears between the loudspeaker and a listener, for example, such as when a meal is served, when a passenger in the next seat goes to the bathroom, or when a listener himself or herself starts to read a magazine or a newspaper, ultrasonic signals are reflected. There are such a variety of conditions under which the ultrasonic signals are reflected, and therefore, it is difficult to individually detect the above-mentioned conditions.
The present invention is to solve the above-mentioned problems, and an object of the present invention is to provide an acoustic apparatus operable to stop or reduce audio in an ensured manner, even when a listener does not intend to stop or reduce the audio, and to cause no sound to be provided to other passengers.